1. Field of the Invention
The present invention relates to an optical glass suitable for mold forming, and particularly relates to an optical glass suitable for mold forming at a low temperature, the optical glass exhibiting a high refractive index and high dispersion property.
2. Description of Prior Art
Currently, the performance requirements for photoelectric products, such as digital cameras, are continually increased, and the dimensions of these products are also required to be further reduced. To meet these requirements, aspheric materials and high refractive index materials must be employed in an optical system design. Within the methods for manufacturing an aspheric lens, the grinding and polishing method not only consumes time, reduces productivity, but increases cost as well. Therefore, the precision press molding technology has now become a major and necessary technology for aspheric lens manufacture.
Japanese Patent Unexamined Publication Nos. 7-247135 and 10-316448 each disclose a molded optical glass having a low glass transformation temperature. The components of the optical glass of Japanese Patent Unexamined Publication No. 7-247135 contain a large amount of lead oxide, PbO. However, precision press molding is ordinarily conducted at a high temperature and in a nonreactive atmosphere or a weakly reducing atmosphere to prevent mold oxidation. When the glass components containing large amounts of lead oxide are precision pressed, lead oxide is reduced on the glass surface and precipitates onto the glass surface as metallic lead. Further, with repeated press molding, the precipitating metallic lead adheres to the molding surface of the mold, decreasing the precision of the molding surface and eventually causing loss of the surface precision of the transfer surface of the molded product. Thus, maintenance is required to remove the metallic lead adhering to the mold, compromising mass production. In addition, the use of poisonous components such as PbO, CdO, As2O3 or TeO2 tends to be avoided lately from optical glasses in view of stringent environmental protection requirements, and thus it is not preferred to use. Japanese Patent Unexamined Publication No. 10-316448 discloses an optical glass containing Tellurium Dioxide, TeO2, that is also poisonous to the human body and therefore it is advisable to avoid the use of TeO2 in the glass component.
In the case of known mold materials for precision molding, there arises a problem that the higher is the precision molding temperature, the more is oxidation or deterioration of the mold material, thus resulting in difficulty in maintenance of the surface accuracy of the mold material and in mass production of lenses by the precision molding. Accordingly, to effectively increase the service life of a mold, the precision molding temperature should be reduced.
On the other hand, the precision molding is generally carried out at a high temperature range of about 30-60° C. above the yield point temperature (At) of the glass. When the yield point temperature of the glass exceeds 590° C., the press temperature becomes 620° C. or greater. Thus, OH material adhering to the surface of the glass reacts with the mold material and ends up decomposing. This decomposition reaction leaves numerous bubbles on the surface of glass lenses that are formed by press molding. Thus, not only does it become difficult to maintain the degree of precision of the transfer surface of the optical part being precision molded, damage is done to the surface precision of the mold material, compromising mass production. Therefore, to ensure mass production, the yield point temperature of the glass should be as lower as possible and at least lower than 590° C.
Among the optical glasses currently available on the market, there are various high refractive index, high dispersion glasses that are disclosed in, for example, Japanese Patent Laid-open Nos. 2001-058845, 2002-173336 and 2003-160355. The high refractive index, high dispersion glass disclosed by Japanese Patent Laid-open No. 2001-058845 has a high liquid phase temperature, and the glass viscosity during precision molding is so low that texture grows in the resultant glass. Further, when manufacturing an optical glass having a high refractive index and high dispersion, there are unavoidable problems in that the glass devitrifies during the manufacturing process and the yield drops. In particular, when employing the reheating press method, devitrification of the glass is marked. This devitrified glass is unsuitable for use in optical elements such as lenses. The high refractive index, high dispersion glass disclosed by Japanese Patent Laid-open No. 2003-160355 has a strong tendency to devitrify or lose transparency near its softening point, thus making it difficult to manufacture glass performs for precision pressing and rendering this glass unsuitable for precision pressing.
Table 1 that is given below shows the glass components, the refractive index (Nd), the Abbe number (vd), the glass transformation temperature (Tg), the yield point temperature (Ts) and the liquid phase temperature (LT) for optical glasses that are disclosed in Japanese Patent Publication Nos. 2002-173336, 2001-058845 and 1996-157231.
TABLE 1No.JP 2002-173336JP 2001-58845JP 1996-157231Weight Percentage (wt %)ComponentP2O519.1123.0027.80B2O32.340.000.00Li2O2.413.002.00Na2O4.178.007.30K2O1.900.000.00SrO1.400.000.00BaO6.196.005.00ZnO3.840.002.60Bi2O30.0010.000.00TiO20.000.006.60Nb2O50.000.0039.80WO331.3238.005.00GeO227.3212.000.00Ta2O50.000.000.00Sb2O30.000.000.00nd1.847031.849131.8442Characteristic Valuesνd23.9323.5521.40Tg471.8472.3552Ts522.8510.7602LT955965—DevitrificationYesNo—
From the above Table 1, it can be seen that the optical glass of Japanese Patent Laid-open No. 2002-173336 loses transparency near its softening point; has a high liquid phase temperature; and thus may develop undesired textures therein. The optical glass of Japanese Patent Laid-open No. 2001-058845 also has a high liquid phase temperature, and thus develops undesired textures therein. The optical glass of Japanese Patent Laid-open No. 1996-157231 has a high glass transformation temperature, which is adverse to the service life of a precision molding mold.